In november 2004, I started a post-doctoral fellowship sponsored by the Japanese
Society for the Promotion of Science (JSPS) at
This laboratory, directed by Prof. Satoshi KAWATA 河田 聡 (see his homepage here)
is located in the main RIKEN - 理化学研究所 (The Institute of Physical and Chemical Research)
institute in the Japanese city of Wako (Saitama prefecture), near by Tokyo. I belong to the Metamaterials sub-team, with Dr. Takuo TANAKA (田中 拓男), Dr. Nobuyuki TAKEYASU (武安 伸幸) and Atsushi ISHIKAWA (石川 篤).
The goal of my current research is to develop three-dimensional (3D) metamaterials, i.e. engineered composites that gain their electromagnetic properties from their inner structures rather than inheriting them directly for the materials they are composed of.
This term is particularly used when the resulting material has properties not found in naturally-formed substances.
In order for the metamaterial to behave as a homogeneous material accurately described by an effective refractive index (effective permittivity and permeability), its structures must have dimension and spacing smaller than the wavelength of operation.
To provide high interaction with the electromagnetic field, metamaterials are composed of highly conducting objects usually made of noble metals such as gold, silver or copper.
Several groups have already reported on the realization and characterization of two-dimensional metamaterials, however innovative fabrication techniques are still lacking to develop truly 3D metamaterials with complicated and fine features. This is the reason why we have proposed a new approach based on a two-photon photopolymerization (TPP) technique combined with selective metal coating by electroless plating to realize such materials.
Electroless plating is a chemical reaction based on the reduction of metal ions in an aqueous solution (silver nitrate in water in our case) upon the action of a reducing agent. The reaction works without the use of electrical energy and allows uniform coating over complex shaped objects. The entire fabrication protocol is described below:
(1) A hydrophobic coating of the glass substrate is realized. (2) One drop of chemically modified photopolymerizable resin is deposited on the substrate. (3) Polymer structures are fabricated by two-photon polymerization. (4) The unsolidified resin is removed with ethanol. At this point, 3D polymer structures with arbitrary shapes can be designed with sub-diffraction resolutions (Fig.1).
(5) A pre-treatment with tin is applied to improve metal deposition and adhesion. (6) Metal coating by electroles plating. (7) Washing of the sample. At this point, all the polymer surfaces are covered by a thin metallic film while the substrate remains uncoated for possible observation in transmission mode (Fig.2).
Our optical setup used for TPP has one particularity; it takes advantage of a 50x50 micro-lens array (Fig.3) to generate multiple beams for parallel fabrication. The lenses are 300 µm in diameter and are arranged in a square lattice with a lattice constant of 300 µm. Their focal length and numerical aperture are respectively 1.5 mm 0.07. TPP is initiated inside the resin with a pulsed Ti:sapphire laser operating at 76 MHz (100 fs pulse width, 799 nm wavelength). To provide enough energy per pulse at each focalization spot, the laser is amplified by a regenerative amplifier. This results in 138 fs pulse at a 1 kHz repetition rate. The multiple beams are focused into the resin matrix through a microscope glass slide, via an inverted oil-immersion objective (60X, NA = 1.4).
High efficiency by parallel processing was demonstrating with the realization of large samples (Fig.3b) made of polymeric rods.
To confirm the presence of silver we have carried out an energy dispersive energy X-ray (EDX) spectroscopy analysis (Fig.4a). The oxygen and silicon peaks originate from the glass substrate, the carbon peak from polymer and the silver peak (Lα1 and Lβ1 lines) results from the electroless plating.
An EDX image recorded at the energy of the main silver peak confirms that silver deposition occurred only at localized positions (bright regions), onto the polymer structures, and that the substrate remains uncoated. The conductivity was evalauted on long polymer lines (Fig.4b) with length ~600-900 µm and width ~1 µm. The lines were coated with a 100 nm thick silver film. Electrical measurements indicate that these lines are highly conductive, with typical resistivity ρ ~ 8.2 ×10-8 Ωm (as compared to 1.6 ×10 -8 Ωm for bulk silver). Fig.4b also shows the dependency of the electrical resistance as a function of length, measured on 1 mm wide macroscale lines, and using different silver nitrate concentration for the electroless plating. The linear behavior of the curves indicates a uniform coating over long distance.